Zipper method for adding chart values

zipper.js

/** Tested in node v0.10.34
 ** This will probably fail in older versions of IE, because IE.

function zipper(/** someArrays, {key:name,value:name} **/) {
  var args = Array.prototype.slice.call(arguments), 
      arrs = args.slice(null, args.length - 1),
      item = args.slice(-1)[0], 
      keys = [];


  if ('[object Object]' !== Object.prototype.toString.call( item )) throw 'Last Argument needs to be an object {key:name,value:name}';
  

  // We need an array of unique keys
  function uniq(array) {
    var ret = [];

    array.forEach(function(k) {
      if (ret.indexOf(k) < 0) {
        ret.push(k);
      }
    });

    return ret;   
  }


  // Get all keys
  arrs.forEach(function(arg) {
    arg.forEach(function(datum) {
      keys.push(datum[item.key]);
    });
  });


  return uniq(keys).map(function(key) {
    var retn = {}, 
        vals = [];

    arrs.map(function(array) {
      return array.filter(function(object) {
        return key === object[item.key];
      })
      .map(function(array) {
        vals.push(array[item.value]);
      });
    });

    retn[item.key] = key;
    retn[item.value] = vals.reduce(function(cur, prev) {
      return cur + prev;
    }, 0)

    return retn;
  });

}